In many applications there is a need for so-called Large Area Detection (LAD) of individual x-rays with high efficiency and high spatial resolution. Charge Coupled Devices (CCDs) are widely used together with a scintillator which converts the x-rays into visible light which can be detected by the CCD. The scintillator material can be for example Gd2O2S or CsI. CsI has the advantage that it can be grown in columnar form, the columns guiding the light in the right direction and reducing the light spread that occurs in a granular scintillator such as Gd2O2S. X-ray detecting devices can operate in different ways, for example by integrating the collected signal over a certain time period to produce an image. Another option is to detect each individual x-ray as a separate event.
Miyata et al. report a photon counting detector to be used as the focal plane of a space telescope. They use a columnar CsI scintillator directly coupled to a CCD for operation in a wide energy band. Low energy photons are directly absorbed in the CCD, while higher energy photons are first converted to visible light in the scintillator, and this visible light is in turn imaged by the CCD. It is however not possible to attain high efficiency simultaneously for all the different energies which can be detected with this device. They use the central pixel of a local maximum as the position of the interaction event, which limits the obtainable resolution depending on pixel size. The pixel size is in turn limited by the fact that smaller pixels mean the signal will be spread over a larger number of pixels and this means a lower value per pixel, which becomes increasingly difficult to distinguish from the noise for smaller pixel sizes.
To avoid the problem that the pixel size puts a severe limit on the spatial resolution, it is possible to estimate the position of interaction based on a centroid estimation of the pixels surrounding a local maximum. Miller et al. report an electron multiplying CCD (EMCCD) with columnar CsI scintillator used in photon counting mode. They perform centroid estimation on detected scintillation events to improve spatial resolution. The CCD images are background corrected and then thresholded slightly above noise level. Miller demonstrates that smoothing of the images prior to the thresholding is not necessary as is done by many others e.g. de Vree et al. and Teo et al. Contiguous regions of pixels above the noise level are identified as an event and centroid estimation is performed on a window around the identified event. While this procedure works well the use of a CCD will still suffer from challenges since the spatial resolution will drop if the efficiency is high using a thick scintillator and also the device will be impractical for low rate, large area applications since, at least in principle, the whole CCD has to be read out to identify events from x-rays, and the thresholding to remove noise is not integrated in the CCD itself but resides on external electronics.
Some of the drawbacks which come from the use of CCD's are outlined more in detail in U.S. Pat. No. 5,912,942, Schick et al. After the CCD has been exposed to the incident light, the pixels are read out in a process where only the pixels on the edge are read out, each capacitor then transfers its contents to its nearest neighbor and the readout process is repeated in this way until all of the pixels in the array have been read out. During this procedure all the CCD pixels are always read out, for large area detectors this produces unmanageable amounts of data. An alternative to a CCD is a CMOS circuit where, contrary to the CCD, each pixel is read out individually and it is therefore possible to employ triggers for events, noise suppression and other information processing. A CMOS circuit coupled to a CsI scintillator is therefore an advantageous solution, as disclosed in U.S. Pat. No. 7,173,251, Fraser et al.
In spite of the progress in the field the last decade there are still no commercial detectors combining semiconductor readout and a scintillator for x-ray conversion with typical resolution for single x-rays better than 50 μm in combination with high efficiency for x-rays with energy exceeding around 20 keV.